Prior to the invention, fiber optic chemical sensor systems have been used to measure the total amount of hydrocarbon dissolved in water. Such systems find useful application in field operations, since the systems can be made portable. Commercial fiber optic chemical sensor systems generally comprise a fiber optic element, a source of light providing light through the core of the element, a detector for sensing changes in the light transmitted through the fiber optic element and for generation of signals, and an analyzer for the signals, the analyzer normally comprising of a computer and appropriate software for processing the signals. At least some of the systems rely on proprietary claddings or coatings on the optic fiber which produce a controlled leakage of light which is a function of the refractive index of the surrounding medium. In one such fiber optic chemical sensor system, a fluorescent tip formed of a fluorescent dye immobilized on the tip of the fiber is provided, an excitation signal is transmitted through the fiber to the tip, and the fluorescent emission is detected through the fiber. In a second type of sensor system, a reflective tip is formed at the end of the fiber so that incident light is transmitted back. In yet a third group, the light source and the detector are positioned at opposite ends of the optic fiber so that variations of the intensity of the light transmitted through the core are detected.
However, as those skilled in the art are aware, the analysis of dissolved hydrocarbon is complicated by the reality that each species of hydrocarbon has its own specific response factor. For this reason, until recently, as indicated in U.S. Pat. No. 5,026,139, fiber optic chemical sensor systems have been generally limited to a single chemical species or physical parameter. Thus, while measurement of total hydrocarbon content in water with such systems has been achieved with a reasonable degree of accuracy, speciation or definition between classes of mixtures with the systems has proven more difficult. Accordingly, prior to the invention, fiber optic chemical sensor technology relied substantially on either thorough knowledge of the source of the water sample, or on additional analytical techniques, generally requiring laboratory facilities, e.g., gas chromatography, to determine hydrocarbon species or a class of hydrocarbons dissolved in water. There has therefore existed a particular need for a technique for differentiating among fuel types dissolved in water, particularly a procedure or method readily adapted for use in a field environment. The invention addresses this need.